Heat exchangers having brazed tube-to-fin joints and methods of producing the same

ABSTRACT

Heat exchangers and methods of producing thereof having fins with slots formed therethrough, and a continuous tube having parallel tube runs connected by reverse bends to define a serpentine coil that traverses back and forth through the slots formed in the fins. Each fin has surface enhancements and is metallurgically joined to corresponding portions of the tube at the slots with brazed joints therebetween.

BACKGROUND OF THE INVENTION

The present invention generally relates to brazing and heat exchanger technologies. The invention encompasses methods of producing heat exchangers by brazing serpentine round tubes to a plurality of fins.

Conventional round tube and plate fin heat exchangers of the type commonly found in air conditioning, and conventional serpentine round tube and plate fin heat exchangers of the type commonly used in household refrigerator evaporators, generally include tubes and fins joined only by mechanical interference fit tube-to-fin joint (“mechanical joints”). This often results in relatively weak tube-to-fin joints which lead to common shortcomings including an overall weak structure, limits to fin density, limits on enhancements to fins, limits on heat exchanger size, and poor surface contact resulting in high thermal resistance. In addition, conventional air conditioning type tube-fin heat exchangers using hairpin-shaped tubes (“hairpins”) and U-shaped return bends (“U” bends) have a relatively large number of tube brazing joints between tube sections resulting in a high potential for refrigerant leakage. Direct refrigerant leakage from HVAC&R systems constitutes a significant quantity of greenhouse gas emissions. Serpentine round tube heat exchangers using “dogbone” type plate fins (fins with elongated “dogbone”-shaped slots) have a relatively small number of tube brazed joints but lack fin enhancements and require mechanical joints, and cannot be used in air conditioning and larger refrigeration equipment, for example, due to weak structure and poor performance.

In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with the prior art, and that it would be desirable if an improved heat exchanger were available that was capable of at least partly overcoming or avoiding these problems, shortcomings or disadvantages.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides heat exchangers and method of producing the same that include brazed tube-to-fin joints with a significantly reduced number of tube brazed joints, increased strength, and increased fin contact area relative to conventional heat exchangers with mechanical interference-fit tube-to-fin joints.

According to one aspect of the invention, a heat exchanger is provided that includes fins having “dogbone”-shaped slots formed therethrough that each defines one or more circular portions interconnected with and intersected by a rectilinear portion that has a width that is narrower than a diameter of the circular portions. Each circular portion has an incomplete circular perimeter and a collar bordering the incomplete circular perimeter. At least one tube is provided having reverse bends forming at least two parallel tube runs to define a serpentine coil. The tube traverses back and forth through the slots formed in the fins. The perimeters of the collars are metallurgically joined to corresponding portions of the tube with brazed joints, and the fins each comprise surface enhancements located between the slots thereof and located along the rectilinear portions thereof.

According to another aspect of the invention, a method of producing a heat exchanger includes the steps of providing fins having “dogbone”-shaped slots formed therethrough such that each slot defines a one or more circular portions interconnected with and intersected by a rectilinear portion therebetween that has a width that is narrower than a diameter of the circular portions. Each circular portion has an incomplete circular perimeter and a collar bordering the incomplete circular perimeter. The fins each comprise surface enhancements located between the slots thereof and located along the rectilinear portions thereof. The fins are assembled with at least one tube having reverse bends forming at least two parallel tube runs to define a serpentine coil such that the tube traverses back and forth through the slots formed in the fins, wherein either the fins or the tube is formed of a material clad with a braze material, and then performing a brazing operation on the fins and the tube such that the collars are metallurgically joined to corresponding portions of the tube with brazed joints formed by the braze material.

Another aspect of the invention is where a heat exchanger as described above comprises a hole in at least a first of the reverse bends and a connection fluidically coupled to the hole is configured to either feed a fluid into or discharge the fluid from a pair of circuits of the tube coupled to opposite ends of the first reverse bend.

Technical effects of heat exchangers and methods of producing the same as described above preferably include the ability to provide heat exchangers with improved tube-to-fin joint strength by utilizing a continuous serpentine tube-type heat exchanger with fins having surface enhancements that can perform as well as or better than current state-of-the-art HVAC&R, hairpin-type tube or serpentine type tube heat exchangers. Such high-performance serpentine tube-type heat exchangers have brazed tube to fin joints and a significantly reduced number of tube brazed joints, resulting in increased heat exchanger strength and reduced risk of refrigerant leakage. Such a design has the potential to reduce the number of brazed joints by more than 70% while maintaining thermal performance comparable to conventional heat exchangers with mechanical joints.

Other aspects and advantages of this invention will be appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For convenience, consistent reference numbers are used throughout the drawings to identify the same or functionally equivalent elements, but with an alphabetical suffix (A, B, or C, etc.) added to distinguish the particular embodiment from other embodiments of the invention.

FIGS. 1 and 2 schematically represent nonlimiting first and second embodiments of heat exchangers comprising one or more serpentine tubes in accordance with certain aspects of the invention.

FIG. 3A schematically represents fluid flow in a prior art heat exchanger, FIG. 3B schematically represents fluid flow in a heat exchanger that utilizes a serpentine tube in accordance with a nonlimiting aspect of the invention, and FIG. 3C represents a detailed view of an optional split adapter used in the construction of the heat exchanger configuration of FIG. 3B.

FIGS. 4A through 4D represent methods of producing holes in round bends of a serpentine tube in accordance with certain aspects of the invention.

FIG. 5A represents locations of multiple brazed joints in a prior art non-continuous “hairpin” tube type heat exchanger. Each hairpin has a mating return “U” bend resulting in two brazed connections for each hairpin. FIG. 5B represents a heat exchanger that utilizes a continuous serpentine tube having brazed joints at only the inlet and outlet thereof in accordance with a nonlimiting aspect of the invention.

FIGS. 6A and 6B schematically represent serpentine tubes having different degrees of orientation in accordance with nonlimiting aspects of the invention.

FIG. 7 represents tube flow configurations for heat exchangers equipped with serpentine tubes in accordance with nonlimiting aspects of the invention.

FIG. 8 represents partial views of two fins having surface enhancements and different types of dogbone-type slots formed therein for receiving a serpentine tube in accordance with nonlimiting aspects of the invention.

FIG. 9 schematically represents plan and cross-sectional views of a portion of either fin of FIG. 8 and a collar formed on the fin to surround a circular portion of the slot in the fin.

FIGS. 10A and 10B represent methods of assembling serpentine tubes with fins having the types of slots represented in FIG. 8.

FIG. 11 shows images of brazed tube-to-fin joints of a serpentine heat exchanger produced using a brazing method in accordance with certain aspects of the invention.

FIG. 12 shows images of mechanical interference-fit tube-to-fin joints of a prior art non-brazed serpentine heat exchanger.

FIG. 13A represents partial plan and end views of a prior art fin configured for receiving an individual round tube of a prior art heat exchanger, and FIGS. 13B and 13C represent partial plan and end views of two nonlimiting enhanced fins for receiving a portion of a serpentine tube in accordance with nonlimiting aspects of the invention.

FIG. 14 represents plan and end views of a fin with surface enhancements having a non-limiting circular portion with a 100-degree perimeter in accordance with another nonlimiting aspects of the invention.

FIG. 15 is a graph plotting simulation results of a continuous serpentine tube-type heat exchanger of the type represented in FIGS. 1 and 2, compared to a baseline air conditioning heat exchanger conventionally formed with hairpin-style tubes and fins with surface enhancements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 5A schematically represents typical locations of multiple brazed joints in a noncontinuous serpentine tube 14A of a prior art non-continuous “hairpin” round tube-type heat exchanger. The serpentine tube 14A comprises hairpins 46 and U-shaped tube sections (U-bends) 48. Each hairpin tube 46 is inserted into a stack of fins (not shown) with round holes and collars. The tubes 46 are then mechanically expanded to interface and contact the collars. To complete the liquid circuitry, the U-bends 48 are brazed to ends of each hairpin tube 46 to form multiple brazed joints 47 as shown. Each return U-bend 48 results in two brazed joints 47. FIG. 5B represents a single continuous serpentine tube 14B capable of use in heat exchangers in accordance with nonlimiting aspects of this invention. In contrast to the non-continuous serpentine tube 14A produced from the hairpin tubes 46 of FIG. 5A, the continuous serpentine tube 14B of FIG. 5B only requires brazed joints 17 at its inlet and outlet. Various embodiments of the invention will be described below as incorporating a continuous serpentine tube of the type represented in FIG. 5B, in part because the ability to reduce the number of brazed joints 17 reduces the likelihood of refrigerant leaks.

FIGS. 1 and 2 schematically represent nonlimiting examples of heat exchangers 10A and 10B that utilize aspects of the continuous serpentine tube 14B of FIG. 5B. Each heat exchanger 10A and 10B comprises one or more round continuous serpentine tubes 14, each having multiple reverse (180 degree or U-shaped) bends (elbows) 16 and parallel straight tube runs 18. In the nonlimiting embodiment of FIG. 1, multiple continuous serpentine tubes 14 are used to define multiple discrete fluid circuits of the heat exchanger 10A, and the tubes 14 are connected to define what may be referred to as a serpentine coil. In the nonlimiting embodiment shown in FIG. 2, a reduced number of brazed joints is achieved with a single continuous serpentine tube 14 that defines a serpentine coil and multiple discrete fluid circuits of the heat exchanger 10B. In large heat exchangers such as found in large AC or refrigeration units, for example, the multiple discrete fluid circuits of the heat exchangers 10A and 10B offer the advantages of additional circuitry connections and reduced pressure drop as compared to a long continuous serpentine tube of the type represented in FIG. 5B. In both embodiments, the tubes 14 traverse back and forth through series of slots (not shown) formed in a plurality of parallel fins 12, and each fin 12 is metallurgically joined with a brazing material to corresponding tube runs 18 of the tubes 14 to define brazed joints at the slots of the fin 12. The heat exchangers 10A and 10B are configured to transfer heat between a fluid (hereinafter referred to as a liquid as a matter of convenience) flowing through their respective coils with another fluid (hereinafter referred to as a gas as a matter of convenience) flowing through the stack of fins 12 (for example, in a direction perpendicular to the plane of the images of FIGS. 1 and 2) for the purpose of heating or cooling the liquid and/or gas. The flow directions of a liquid through the tubes 14 are represented with arrows in FIGS. 1 and 2.

The continuous serpentine tube-type heat exchangers 10A and 10B of FIGS. 1 and 2 may be assembled by inserting a complete serpentine tube 14 into a stack of fins 12 each having elongated “dogbone” shaped slots (i.e., slots having a pair of generally circular portions and a rectilinear portion therebetween that is narrower than the circular portions). In this case, the leading edge of the serpentine tube 14 is narrowed (e.g., flattened slightly) to allow the serpentine tube 14 to be inserted through the slots in the fins 12. As discussed in greater detail below, the fins 12 of the continuous serpentine tube-type heat exchangers 10A and 10B are also formed to have surface enhancements at locations along and between their dogbone slots to promote the heat transfer efficiency of the heat exchangers 10A and 10B.

As described in more detail below, the heat exchangers 10A and 10B comprise certain features and aspects that are believed to provide comparable or improved performance relative to conventional heat exchangers that have serpentine round tubes and non-enhanced fins, or have hairpin-tubes with enhanced fins that are joined mechanically to yield what are referred to herein as mechanical interference-fit tube-to-fin joints, or more simply mechanical joints. Exemplary improvements are represented in FIG. 15, which shows simulation results of a continuous serpentine tube-type heat exchanger of the type described above in reference to FIGS. 1 and 2, compared to a baseline air conditioning heat exchanger conventionally formed with hairpin-style tubes and fins with surface enhancements. The heat exchangers 10A and 10B may provide improvements for a wide variety of applications including air conditioning, refrigeration, heat pumps, and other devices that use a heat exchanger to transfer heat between a gas and a liquid.

FIG. 1 represents an example of a joining configuration for serpentine tube heat exchangers where parallel circuitry is desired in order to reduce liquid pressure drop, wherein each tube 14 of the serpentine coil individually has a connection for receiving the liquid and a connection for discharging the liquid from the discrete fluid circuit defined by the tube 14. The heat exchanger of FIG. 2 represents an example of advantageously reducing the number of brazed joints as compared to the heat exchanger 10A of FIG. 1 by using a split joining configuration wherein multiple circuits (in FIG. 2, pairs of adjacent circuits) receive the liquid through a shared split connection 19A and multiple circuits (in FIG. 2, one pair of adjacent circuits) discharge the liquid through a shared split connection 19B, thus resulting in fewer overall feeding/discharging connections required. The split joining configuration of FIG. 2 provides for a significant reduction in joints per circuit and a potential reduction in refrigerant pressure drop and further reduced risk of refrigerant leakage.

FIG. 3A schematically represents a row of prior art hairpins 46 prior to their assembly and brazing to return “U-bends” to yield a discontinuous serpentine tube with multiple braze joints. In contrast, FIGS. 3B and 3C schematically represent a continuous serpentine tube 40 with adjacent bends, one of which is equipped with a straight tube section 42 forming a split joining configuration that can function as an inlet or outlet (indicated as an outlet in FIG. 3B). FIG. 3C schematically represents an isolated view of a bend of the serpentine tube 40 that was produced to include the straight tube section 42 during investigations leading to the present invention. To accept the tube section 42, a hole was formed in the bend via electrical discharge machining (EDM) and then expanded to form a collar (not shown) to accept the tube section 42. The tube section 42 was then partially inserted into the hole so that the collar firmly held the tube section 42 in place prior to brazing.

As the method of FIGS. 3B and 3C effectively forms a split connection, the technique can also be used to form the split connections of a continuous serpentine tube similar to that of FIG. 2. That is, EDM may be used to provide a hole directly in one or more of the bends 16 of the continuous serpentine coil, and the holes may undergo additional forming to produce collars into which straight tube sections 42 may be inserted. The straight tube sections 42 and bends 16 are then metallurgically joined with brazed joints formed during a subsequent brazing process.

Manufacturing an integral serpentine tube 14 for the split joining configuration of FIG. 2 may be a challenge with respect to tools, manufacturing order, shape, and sizes. Conventional drilling is preferably avoided due to the risk of depositing metal dust and debris inside the tube 14. Therefore, techniques such as puncturing, piercing, and EDM cutting are preferably used. FIGS. 4A through 4D represent four methods of providing holes in a bend of a serpentine tube (such as the U-shaped bends 16 of the tube 14 in FIG. 2). FIG. 4A depicts a first method in which partial holes are cut in edges of a sheet before rolling the sheet into a tube, which can then undergo bending to form a U-shaped bend. FIG. 4B depicts a second method in which a hole is punched in a tube after the tube has been bent to form a U-shaped bend. Preferably, the bend is punctured to form the hole after the tube has been brazed to fins to avoid hole tapping with any cladding material on surfaces of the tube 14. A particular challenge for this method is having a tool that fits within the confined space between the inside of the bend and the fins. FIG. 4C depicts a third method in which a round hole is punched in a straight round tube before the tube is bent to form a U-shaped bend. Puncturing the tube is relatively easy, but the round hole becomes elliptical once the tube is bent to form the bend. Additionally, hole locations need to be precisely calculated to match the coil design. FIG. 4D depicts a fourth method in which an elliptical hole is punched in a straight round tube before the tube is bent to form a U-shaped bend, and the hole acquires a rounder shape when the tube is bent to form the bend.

The previously described serpentine tubes 14 (FIGS. 1 and 2), 40 (FIGS. 3B and 3C), and 14B (FIG. 5B) define serpentine coils that essentially entirely lie in a single plane, such that adjacent bends 16 of the coil are oriented at an angle θ of 0 degrees, as represented by the tube 14 in FIG. 6B. Alternatively, adjacent bends 16 of the serpentine coil may be oriented at a corrugation angle θ other than 0 degrees (slanted) such that the coil “zigzags” back and forth between what can be described as separate banks of tubes that lie in separate but parallel planes, as represented by the tube 14 in FIG. 6A. In the example of FIG. 6B, dogbone slots formed in fins to be assembled with the tube 14 are parallel to the longitudinal axis of each fin. In the example of FIG. 6A, dogbone slots formed in fins to be assembled with the tube 14 are inclined at the corrugation angle θ to the longitudinal axis of each fin. The heat exchangers 10A and 10B may include one or more banks of tubes. For a heat exchange comprising multiple banks of tubes, either or both coil configurations of FIGS. 6A and 6B may be used. As represented in FIG. 7, the coil configuration of FIG. 6B can be utilized in full-cross counter flow heat exchangers, while the coil configuration of FIG. 6A can be utilized in semi-cross parallel or counter flow heat exchangers.

As noted above, slots are formed in the fins 12 to receive the serpentine tubes 14 or tube runs 18 of the heat exchangers 10A and 10B. The method of assembly will be affected by both the type of serpentine coil used (e.g., FIG. 6A or 6B) and the type of slots provided in the fins 12. According to a preferred aspect of the invention, FIG. 8 represents two nonlimiting types of slots (Type 1 and Type 2) that may be formed in the fins 12 of the heat exchangers 10A and 10B to receive their serpentine tubes 14. The fin 12 identified as Type 1 has a “dogbone” slot 22 aligned parallel to the longitudinal length of the fin 12 and defined by a pair of generally circular portions 24 intersected and interconnected by an intermediate rectilinear portion 26 therebetween that is narrower than the circular portions 24. The fin 12 identified as Type 2 has partial “dogbone” slots 23 each aligned perpendicular to the longitudinal length of the fin 12 and defined by a single circular portion 24 intersected by a rectilinear portion 27 that is narrower than the circular portion 24 and defines an opening at a downstream edge of the fin 12. As a result of intersecting the rectilinear portion 26 or 27, the circular portions 24 of the slots 22 and 23 do not have complete circular perimeters, but instead each circular portion 24 has an incomplete circular perimeter that spans an arc of greater than 180 degrees. According to a preferred aspect of the invention, the circular sections 24 are not required to form mechanical interference-fit joints with the tubes 14 with which they are assembled. Instead, and as schematically represented in FIG. 9, the circular portions 24 of the slots 22 and 23 can be sized to define a radial gap 33 between the tube 14 and fin 12 of greater than 0 mm up to about 0.1 mm, for example, from about 0.05 to 0.1 mm, which must be bridged by a brazed joint. In another embodiment, a Type 2 fin slot 23 may be complete and similar to a Type 1 slot 22. In such embodiment, the Type 2 fin is similar to Type 1 except that the slot orientation is perpendicular to the Type 1 slot.

As also represented in FIG. 9, a collar 32 borders at least a portion of, and preferably the entire, incomplete circular perimeter of each circular portion 24 of the slots 22 and 23, so that an inner surface of the collar 32, and not just the inner edge of the circular portion 24, faces the outer circumference of the tube 14 and is available as a heat transfer path between the tube 14 and fin 12. The collars 32 can be formed by material of the fin 12 that has been bent away from adjoining surfaces of the fin 12. As represented in FIG. 9, the collars 32 may be preferably formed to have a reflare section 34, defined herein as a surface that bends away from the central axis of the collar 32 and the circular portion 24 it borders. The reflare sections 34 of the collars 32 allow the fins 12 to be stacked one against the other without the fins 12 telescoping into each other, which allows for the assembly of heat exchangers 10A and 10B with increased fin densities relative to, for example, conventional refrigerator low-fin density evaporators using serpentine tube designs.

FIG. 10A schematically represents the manner in which a serpentine tube 14 can be inserted into a stack of the Type 1 fins 12 of FIG. 8 in the transverse (z-axis) direction of the fins 12. This insertion method is essentially the same as would be employed to assemble a serpentine tube with a stack of conventional fins. However, the interference fit required to create the mechanical joints between a tube and conventional fins requires a relatively high force to insert the tube through a stack of fins. This necessitates the use of fins that are stiff, which discourages the presence of surface enhancements on the fins. In contrast, the gap 33 between the fins 12 and tube 14 of this invention reduce the force required to insert the tube 14 into a stack of the Type 1 fins 12 of FIG. 8. As a result, the fins 12 can be equipped with surface enhancements 36 capable of significantly increasing the thermal efficiency of the fins 12. Investigations leading up to the present invention showed that gap 33 between the fins 12 and a tube 14 of up to about 0.1 mm can be easily bridged by a braze material.

For the Type 2 fin 12 of FIG. 8, the same assembly technique cannot be performed without modifications to the serpentine tube 14. FIG. 10B represents two methods of inserting serpentine tubes 14 into a Type 2 fin 12 of FIG. 8. One method involves lateral insertion of the tube 14 in the x-axis direction of each fin 12, and the other method involves bending the series of the bends 12 on one end of the tube 14 so that it can move through the slots 23 in the transverse (z-axis) direction of each fin 12. In these figures, solid white arrows indicate the directions of tube insertion. The serpentine tube lead area is slightly flattened to allow the tube 14 to penetrate the “dogbone” shape slot 23. For example, the tube leading bends in FIG. 10A are slightly flattened to allow penetration of the serpentine tube 14 through the rectilinear portion 27 of the slot 23.

Once the tubes 14 and fins 12 have been assembled and prior to brazing, the entire heat exchanger assembly may be de-greased to remove process oils if needed. Then the entire heat exchanger is placed in a furnace and heated at appropriate temperatures and durations to form complete brazed joints between the tubes 14 and fins 12. The components of the heat exchangers 10A and 10B may be formed of various materials suitable for brazing and producing brazed joints between the tubes 14 and fins 12. For example, either the tubes 14 or fins 12 may be formed of a material having a cladding material thereon that contains a source of the braze material. As a nonlimiting example, the tubes 14 may be formed of an aluminum alloy having a clad layer formed of a 4000-series aluminum-silicon alloy as the braze material. Such alloys typically contain about 10 to 12% silicon and the clad layer thickness accounts for about 10% of the wall thickness of the tube 14.

Fluxing of the heat exchanger assembly prior to brazing can be performed using various methods capable of depositing a small controlled amount of flux on the components of the assembly. For example, one such method involves mixing a flux powder with isopropyl alcohol and spraying the resulting flux mixture on the assembly while the flux powder is kept in suspension. Another such method involves pouring a mixture of a flux powder and water on the assembly. After the flux mixtures are applied, the heat exchanger assembly is preferably dried, for example, using forced hot air or in a drying furnace.

Preferably, the brazing operation is performed in a furnace that contains an inert atmosphere, for example, nitrogen gas, to reduce the likelihood of oxidation of the alloy from which the tubes 14 are made. A controlled atmosphere brazing furnace (CAB) is particularly well suited for this purpose, though other aluminum brazing equipment and methods may be used. The temperature profile of the brazing operation may include a rapid temperature increase up to the point where the flux melts. The flux removes any oxide layer from the surfaces of the heat exchanger components and allows the cladding material, as the source of the braze material, to properly flow and wet the surfaces to be joined by the braze material. The temperature of the assembly preferably continues to increase to the liquidus temperature of the braze material. Tube-to-fin joints are created as the braze material flows to create the required braze joint fillets. The creation of the fillets is also aided by capillary action of the gap 33 between the tube 14 and fins 12, which is preferably not greater than about 0.1 mm to reduce the risk of forming incomplete fillets if there is insufficient braze material.

The assembly is held at the liquidus temperature of the braze material for a duration dependent on a few factors. For example, the size of the heat exchanger assembly and how homogeneous the temperature distribution is within the furnace. Afterwards, the resulting brazed heat exchanger is moved from the brazing chamber to a cooling zone of the furnace where the brazed heat exchanger is cooled before exiting the furnace. If removal of the dried flux residue is desired, the heat exchanger can be washed.

In an investigation leading to the present invention, sample heat exchanger assemblies were produced using aluminum alloy (Alloy 3003) tubes having 7.2 or 8 mm outer diameters and 0.6 mm wall thicknesses, and an outer clad layer containing an aluminum-silicon eutectic brazing alloy (Alloy 4045) and 1% zinc. The clad layer constituted about 10% of the wall thickness. The tubes were assembled with fins resembling the Type 1 fin represented in FIG. 8 as having a “dogbone” slot and circular portions bordered by collars with a reflare section, as described above in reference to FIG. 9. The reflare sections of the collars were intentionally formed to promote stacking of the fins without the fins telescoping into each other and to allow for the building of heat exchangers 10A and 10B with increased fin densities and homogeneous fin spacing relative to, for example, conventional low-density, no enhancement fin refrigerator evaporators using serpentine tube designs. As also described in reference to FIG. 9, the sizes of the slots and collars of the fins were such that radial gaps of up to 0.1 mm were present between the fins and the tubes assembled therewith.

The samples were coated with a standard aluminum brazing flux. It was observed that even when the fins were stacked closely such that their collars contacted adjacent fins, the flux was able to penetrate and coat surfaces of the tubes and fins within the areas of contact between the tubes and the fin collars. In particular, the flux appeared to penetrate and coat the required area of the collar and tube through the remaining open area within each slot. During the brazing process, the samples were brazed in a nitrogen atmosphere furnace following a temperature profile of a temperature rise up to 605° C., a soak time of about one minute, and cooling to about 150° C., after which the samples were removed from the furnace and air cooled to room temperature. Examination of the brazed samples showed successful brazed joints were achieved with complete fillets inside and around each fin collar, filling the gaps between the collars and tubes. Therefore, it was concluded that the reflare sections of the collars were beneficial to stacking the fins to increase fin densities without the fins telescoping into each other and that flux could penetrate to the required areas to provide a good braze joint.

FIG. 11 shows images of brazed joints of a serpentine heat exchanger produced during the investigation. For contrast, FIG. 12 shows images of mechanical joints of a conventional non-brazed serpentine heat exchanger. Without brazed joints, there are gaps between the tube and fins in FIG. 12, and the gaps can be larger than the fin thickness. In contrast, the brazed joints completely fill the gaps between the tube and fins in FIG. 11. Even where the gap between a fin and the tube was large, the braze material was able to completely bridge the gap. The brazed joint established a much wider contact area that extended from beyond the tip of the “pseudo-collar” to the tip of the braze material between the tube and the “elbow” of the “pseudo-collar.” This wider contact area between the fins and tubes improves the heat transfer and therefore increases the performance of the heat exchanger. This increased contact surface area compensates for the lack of fin surface area due to the large dogbone slot 22, compared with a conventional fin having a circular hole (FIG. 13A).

Torsion tests conducted on brazed and mechanical joint samples such as shown in FIGS. 11 and 12, respectively, indicated that the brazed sample was rigid due to its brazed joints, whereas the mechanical joint sample allowed the tube run to twist within its fins even under a very small torsional load, too small to measure with available equipment.

As noted above, conventional heat exchangers having tubes and fins joined mechanically (mechanical joints) require fins that are relatively stiff in order to prevent the fins from collapsing when a linear tube run or serpentine tube is inserted through the holes in the fins during assembly. In contrast, the fins 12 described above in reference to FIGS. 8 and 9 do not require an interference fit during assembly with a tube. Instead, the gap 33 between the fins 12 and tube 14 assembled therewith provide a sufficient radial clearance of about 0.05 to 0.1 mm, such that the force required to introduce the tube 14 into the fin 12 during assembly is lower than with heat exchangers requiring an interference fit. Therefore, while the brazed heat exchangers 10A and 10B may include smooth planar fins 12 of the types used on conventional heat exchangers, they may also or alternatively include fins 12 having surface enhancements that significantly increase fin efficiency. Such enhancements have been determined to allow fins 12 having dogbone-shaped slots 22 or 23 to perform competitively with conventional fins having circular interference-fit holes despite a reduced surface area due to the presence of the rectilinear portion 26 or 27 of the slot 22 or 23.

FIG. 13A represents a fragment of a fin 12A of a type used in conventional mechanically-expanded tube heat exchangers used in air-conditioners, having multiple hair-pin tubes, multiple brazed CU″ bends, and fully circular (360 degree) holes 20 with louvers 28 between each hole 20 that serve as surface enhancements. FIGS. 13B and 13C represent two nonlimiting embodiments by which surface enhancements can be incorporated into brazed serpentine tube heat exchangers of the type disclosed above (e.g., FIGS. 1 and 2) to improve the performance of the heat exchangers such that they perform equal to or better than conventional air-conditioner heat exchangers. Such enhancements include but are not limited to slits (raised lances), louvers 28 (corrugated lances) and winglets 30 (turbulence generators). FIGS. 13B and 13C represent fragments of fins 12B and 12C having “dogbone” shaped slots 22 and rectilinear portions 26 that subtend an angle of about 120 degrees of their respective circular portions 24. FIG. 13B represents the fin 12B as having eight louvers 28 between the slots 22 and twelve winglets 30 adjacent to the slot 22. FIG. 13C represents the fin 12C as having twelve upper louvers 28A between the slots 22 and eight lower louvers 28B adjacent to the slots 22. The upper and lower louvers 28A and 28B do not necessarily have the same dimensions and characteristics. The end views of each fin 12A, 12B, and 12C show relative angles of the louvers 28. Fins 12 having other enhancement configurations are foreseeable and within the scope of the invention.

FIG. 14 represents a modified version of the fin 12C of FIG. 13C, labeled in FIG. 14 as 12D, wherein the rectilinear portions 26 are narrowed such that they subtend an angle of about 100 degrees of their respective circular portions 24. This modification increases the tube-to-fin contact area at the collar (not shown). The lower louvers 28B also include larger dimensions since there is additional space available. Notably, narrowing the rectilinear portion 26 requires the tube 14 to be further flattened in order to be inserted into the slot 22. As the tube 14 flattens, its cross-sectional area reduces, thus increasing a local flow resistance and resulting in a pressure drop penalty.

Brazing serpentine tube-to-fin joints as described herein provides significantly increased joint strength compared to mechanical joints. This improved strength allows for the production of larger heat exchangers, particularly for air-conditioning and large refrigeration systems which were previously limited in size due to mechanical joints in the prior art heat exchanger. The brazed joints also provide increased surface contact area and therefore increased thermal conductivity between tube and fin thus improving heat exchanger performance. Brazed joints also allow for the introduction of surface enhancements like louvers on the fin surface since little pressure is exerted on the fins during insertion into the serpentine tube.

While the invention has been described in terms of a specific or particular embodiment, it should be apparent that alternatives could be adopted by one skilled in the art. For example, the heat exchanger 10 and its components could differ in appearance and construction from the embodiment described herein and shown in the drawing, functions of certain components of the heat exchanger 10 could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, various materials could be used in the fabrication of the heat exchanger 10 and/or its components, and the heat exchanger 10 could be installed in various types of heating, cooling, or electrical systems. In addition, the invention encompasses additional or alternative embodiments in which one or more features or aspects of a particular embodiment could be eliminated. Accordingly, it should be understood that the invention is not necessarily limited to any embodiment described herein or illustrated in the drawing. It should also be understood that the phraseology and terminology employed above are for the purpose of describing the disclosed embodiment, and do not necessarily serve as limitations to the scope of the invention. Therefore, the scope of the invention is to be limited only by the following claims. 

1. A heat exchanger comprising: fins having dogbone-shaped slots formed therethrough, each of the slots defining one or more circular portions interconnected with and intersected by a rectilinear portion that has a width that is narrower than a diameter of the circular portions, each of the circular portions having an incomplete circular perimeter and a collar bordering the incomplete circular perimeter; at least one tube having reverse bends forming at least two parallel tube runs to define a serpentine coil that traverses back and forth through the slots formed in the fins; and surface enhancements located between the slots thereof and located along the rectilinear portions thereof; wherein the collars of the slots are metallurgically joined to corresponding portions of the tube with brazed joints.
 2. The heat exchanger of claim 1, wherein each of the slots defines an entire dogbone shape comprising a pair of the circular portions interconnected with and intersected by the rectilinear portion therebetween.
 3. The heat exchanger of claim 1, wherein each of the collars has a reflare portion that defines a surface that bends away from a central axis of the collar and the circular portion bordered thereby.
 4. The heat exchanger of claim 1, wherein each of the collars defines a gap between the incomplete circular perimeter thereof and the corresponding portion of the tube and the gap is filled by one of the brazed joints.
 5. The heat exchanger of claim 1, wherein each of the slots has only one of the circular portions and the rectilinear portion defines an opening at an edge of the fin.
 6. The heat exchanger of claim 5, wherein each of the fins comprises a second slot having a single circular portion and a single rectilinear portion that defines an opening at the edge of the fin.
 7. The heat exchanger of claim 5, wherein each of the collars has a reflare portion that defines a surface that bends away from a central axis of the collar and the circular portion bordered thereby.
 8. The heat exchanger of claim 5, wherein each of the collars defines a gap between the incomplete circular perimeter thereof and the corresponding portion of the tube and the gap is filled by one of the brazed joints.
 9. The heat exchanger of claim 1, wherein the tube includes at least two adjacent circuits each having a first connection configured to feed a fluid into the circuit and a second connection configured to discharge the fluid from the circuit, wherein at least one of the first and second connections is shared by the at least two adjacent circuits to define a split connection.
 10. The heat exchanger of claim 9, wherein the tube is a continuous tube that defines the serpentine coil in its entirety, and the split connection is located at a corresponding one of the reverse bends in the tube.
 11. The heat exchanger of claim 9, wherein the tube comprises a plurality of continuous tubes that define the serpentine coil.
 12. A method of producing a heat exchanger, the method comprising: providing fins having dogbone-shaped slots formed therethrough, each of the slots defining one or more circular portions interconnected with and intersected by a rectilinear portion that has a width that is narrower than a diameter of the circular portions, each of the circular portions having an incomplete circular perimeter and a collar bordering the incomplete circular perimeter, the fins having surface enhancements located between the slots thereof and located along the rectilinear portions thereof; forming at least one tube having reverse bends and at least two parallel tube runs to define a serpentine coil; assembling the fins and the tube such that the tube traverses back and forth through the slots formed in the fins, the assembling of the fins and the tube including inserting each of the reverse bends through a corresponding one of the slots; and performing a brazing operation on the fins and the tube such that a braze material clad on at least one of the tube and fins melts and the collars of the slots are metallurgically joined to corresponding portions of the tube with brazed joints formed by the braze material.
 13. The method of claim 12, wherein each of the slots defines an entire dogbone shape comprising a pair of the circular portions interconnected with and intersected by the rectilinear portion therebetween.
 14. The method of claim 12, wherein each of the collars defines a gap between the incomplete circular perimeter thereof and the corresponding portion of the tube, and the brazing operation causes the brazed material to fill the gap.
 15. The method of claim 12, wherein each of the collars has a reflare portion that defines a surface that bends away from a central axis of the collar and the circular portion bordered thereby.
 16. The method of claim 12, the method further comprising stacking the fins prior to the assembling of the fins and the tube such that at least some of the fins contact corresponding collars of the fins adjacent thereto.
 17. The method of claim 12, further comprising providing a hole in at least a first of the reverse bends and providing a connection fluidically coupled to the hole configured to either feed a fluid into or discharge the fluid from a pair of circuits of the tube coupled to opposite ends of the first reverse bend.
 18. The method of claim 17, wherein the hole is formed in the first reverse bend after the first reverse bend is formed.
 19. The method of claim 17, wherein the hole is formed in the first reverse bend before the first reverse bend is formed.
 20. The method of claim 17, wherein the hole has an elliptical shape before the first reverse bend is formed and a circular shape after the first reverse bend is formed. 